1. Technical Field
The invention relates to the field of thermal printing. More specifically, the invention relates to thermal printing techniques for producing markings on multiple sides of a thermochromic medium. Particular embodiments of the invention are particularly useful in thermal printing applications to mark votes on a two-sided ballot during an election to provide visual evidence of voters' selections.
2. Description of the Related Art
In general, thermal printing methods produce an image by selectively heating a thermochromic medium using a thermal printer head. Thermal printing is typically practiced by first applying a coating of thermochromic dye to a substrate, resulting in a thermochromic medium. Thermochromic dye is responsive to heat and changes its appearance when heat-activated, resulting in a marking. After an application of dye, the process continues by selectively applying heat to various regions of the thermochromic medium, thus leaving markings on the medium.
However, various problems exist in the field of thermal printing. For example, as heat naturally dissipates, bleeding may occur, thus marking unintended regions of the medium. In particular, a marking on one side of the printing medium may conduct through the substrate, resulting in an unintended marking on the opposite side.
Recent controversies surrounding the accuracy of electoral systems have led to the development of direct recording electronic (DRE) voting machines that, according to proponents, minimize the number of inadvertently spoilt ballots. Critics of DRE machines, however, challenge the resulting lack of a voter verifiable paper trail. Consequently, hybrid systems have been proposed in which an electronic voting machine, e.g. a computer with a touch sensitive display, assists voters in marking a paper ballot that may be inspected by the voter prior to optical scanning. In these systems the ballots are archived should the counting procedure be subsequently contested.
Thermal printing is an attractive technology for use in ballot marking systems on account of the simple and reliable printing mechanism and the relatively low resolution of typical ballot markings. However, as explained above, an unintended marking of a ballot due to the shortcomings of the current state of the art negates the advantages of employing a thermal printing system to record votes.
Furthermore, the majority of existing thermal printing systems are capable of printing only on a single side of each sheet of thermochromic media. Typically, a thermochromic coating is applied to only one side of the substrate. However, when marking printable media, a desire for readability and simplicity strongly motivates printing on both sides of a single sheet of thermochromic media. In the case of a ballot, limiting a printable media to a single sheet simplifies ballot handling and improves the integrity of the election by eliminating the possibility of a split ballot. Yet the length of many ballots, especially those with multiple voter initiatives, makes compressing all election issues onto a single side of a single sheet a difficult endeavor if readability is to not be compromised. The increased surface area available to two-sided printing systems strongly motivates the use of a printing system with two-side printing capability.
Various two sided thermal printing systems have been proposed, but typically either increase the complexity of the heating element control system or require a substrate of sufficient thickness and insulating capability as to prevent heat conduction from one side to the other. However, simpler approaches would be highly desirable for use in ballot marking.
Of course, any thermal printing system must ensure uniform thermal contact between the thermochromic ballot and the heating elements that produce the markings. Indeed, ensuring proper contact in the case of marking ballots presents special challenges. This is because the ballots are necessarily handled by voters themselves, who may or may not be familiar with thermal printing technology. Thus, there is a need for a simple yet reliable mechanism for ensuring uniform contact between the ballot and the heating elements after the ballot is received from the voter.